The invention relates to a pressure limiting valve for the protection of hydraulic power packs against overload, in particular, for the protection of hydraulic props in underground mining and tunnel construction against falling rocks, comprising a valve housing with consumer connection and pressurized fluid outlet, which are separated from one other by means of a movable closure device with a seal securing the flow gap between the pressurized fluid outlet and the consumer connection, which closure device is movable against the force of a valve spring, and are connected to one another if an overload occurs in order to discharge the pressurized fluid.
Pressure limiting valves are used in various hydraulics-related areas, in particular, to protect hydraulic power packs against overload. A valve piston is normally disposed in a valve housing such that in case of overload it passes over a seal and allows the pressurized fluid to flow off through the pressurized fluid outlet until the overload has been reduced and the valve can close again. The seal to be passed over by the closure device of the valve, typically a valve piston movable in a piston bore, is exposed to high loads because the closure device is pressurized by the compressed pressurized fluid, and even so has to remain in a state that ensures overall sealing of the system. As a result of the pressure, the seal is deformed in such a way that it projects into the flow gap between the movable closure device and the fixed component, and therefore is exposed to severe stress at the start of movement of the closure device. Hence, the lifetime of such seals is limited. This applies particularly for the valve described in EP-A-0 096 303.
The O-ring, which can be deformed like plastic mass and is inserted prestressed, is deformed when passing over the radial bores of the valve piston (closure part) such that the flow gap is closed by the developing annular projection. With the valve configuration according to DE-8 437 091 U1 and GB-A-2 159 923 the seal rings are fitted into the respective groove or are inserted prestressed therein in the case of both valve pistons in order to ensure an accurate seat of the O-ring. Here as well the O-ring is pressed into the flow gap, and the developing annular projection creates the afore-mentioned wear problems. The problems described cannot be resolved even the reduced thickness of the O-ring because again the hydraulic pressure occurring in the groove or acting in the groove acts in the direction of the flow gap, and is in fact supposed to act in that direction, because, as is indicated in FIG. 3 of EP-A-0 096 303, this closure is the objective of the deformation. DE-3 909 461 A1 shows among a variety of illustrations in FIG. 3 also such a “reduced” O-ring. The shape and arrangement as well as the material of these known O-rings result in the afore-mentioned disadvantageous deformation and consequently to the premature failure of these pressure limiting valves, which are important for safety reasons.